1. Field of the Invention
The present invention relates to methods of manufacturing a stacked thin film piezoelectric filter including at least two thin film piezoelectric resonators.
2. Description of the Related Art
Inter-stage filters and duplexers are essential for the circuits of wireless communication apparatuses, such as cellular phones, for example. Hitherto, surface acoustic wave (SAW) filters and dielectric filters have been used for such applications. In recent years, thin film piezoelectric filters have been used for inter-stage filters and duplexers in accordance with requirements for a high withstand power, a low loss, and a reduced size. Further, in response to market requirements, thin film piezoelectric filters with a balanced-unbalanced conversion function that allows an unbalanced signal to be converted to a balanced signal have been developed. A method for providing a balanced-unbalanced conversion function in a thin film piezoelectric filter is to stack two resonators in the thickness direction thereof, and thereby couple the mechanical oscillation of the resonators to each other. The structure of such a vertical coupling thin film piezoelectric filter is disclosed in U.S. Pat. No. 6,720,844, for example.
Referring to FIG. 13, the stacked thin film piezoelectric filter of U.S. Pat. No. 6,720,844 includes thin film piezoelectric resonators 121, 122, 123, and 124. The thin film piezoelectric resonators 121 and 122 are acoustically coupled with each other through an acoustic coupling layer 113. The thin film piezoelectric resonators 122 and 123 are electrically connected to each other. The thin film piezoelectric resonators 123 and 124 are acoustically coupled with each other through the acoustic coupling layer 113. A signal input to the thin film piezoelectric resonator 121 is output to the thin film piezoelectric resonator 124 through the thin film piezoelectric resonators 122 and 123. The thin film piezoelectric resonators 122 and 123 are disposed on an acoustic reflection layer 112 that is disposed on a substrate 111.
A known method of adjusting the frequency of the stacked thin film piezoelectric filter of U.S. Pat. No. 6,720,844 is disclosed in Japanese Unexamined Patent Application Publication No. 2007-166148. This document discloses a method of adjusting the resonant frequency by adding a new layer on an upper electrode or by decreasing the thickness of the electrode through etching, for example.
However, when the method disclosed in Japanese Unexamined Patent Application Publication No. 2007-166148 is applied to the stacked thin film piezoelectric filter disclosed in U.S. Pat. No. 6,720,844, although the frequencies of the thin film piezoelectric resonators 121 and 124 can be adjusted, the frequencies of the thin film piezoelectric resonators 122 and 123 cannot be adjusted. Thus, the frequency of the thin film piezoelectric resonator 121 or 124 is different from the frequencies of the thin film piezoelectric resonators 122 and 123, thereby causing the frequency characteristics to be deteriorated.